A conventional multilayer optical interference filter comprises several thin layers of optically transmissive materials. The materials of adjacent layers have different refractive indices. The filter characteristics are determined by the number, order, thicknesses and refractive indices of the layers. The theory of multilayer interference filters is well developed and is explained in several standard optical references, including Principles of Optics (Born and Wolf, Pergamon Press, 1964, pp. 51-70). Multilayer interference filters are commonly used for wavelength or polarization selective reflection or transmission in mirrors, lenses and beam splitters.
While conventional multilayer optical interference filters are generally made by vacuum evaporation of dielectric thin films on a suitable substrate, other materials and fabrication methods have been used. For example, van der Ziel et al, Thornton et al, and Gourley et al report interference filters comprising alternating layers of undoped GaAs and AlGaAs semiconductor formed by molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD) (van der Ziel et al, Applied Optics, Vol. 4, No. 11, November 1975, pp. 2627-2630, Thornton et al, Applied Physics Letters, 45(10), Nov. 15, 1984, pp. 1028-1030, Gourley et al, Applied Physics Letters 49(9), Sept. 1, 1986, pp. 489-491).
U.S. Pat. No. 4,309,670 (issued Jan. 5, 1982, in the names of R. D. Burnham et al) discloses interference filters comprising alternating layers of Ga.sub.1-w Al.sub.w As and Ga.sub.1-y Al.sub.y As where w&gt;y. All of the alternating layers are doped with impurities of the same conductivity type. These interference filters are used as passive reflectors in Distributed Bragg Reflector (DBR) semiconductor lasers.
At the 45th Annual Device Research Conference (Santa Barbara, Calif., June 22-24, 1987, Paper IIIA-4) et al reported that GaAs-AlGaAs multilayer optical interference filters can be made electrically tunable by placing quantum wells in every second layer of the multilayer structure. Application of a large electric field in a direction which is normal to the semiconductor layers of this structure induces changes in the refractive index of the layers containing the quantum wells by a mechanism known as the "quantum confined Stark effect". The induced refractive index changes tune the interference filter, shifting the wavelengths at which maximum reflection and maximum transmission occur. Unfortunately, this electrically tunable interference filter requires large driving voltages (approximately 100 V). Moreover, the large driving voltages induce significant leakage currents. The heat generated by the leakage currents changes the device temperature locally, causing local perturbations of material parameters such as refractive index and producing unwanted changes in the filter characteristics. The generated heat also degrades device reliability.
Katz et al report multilayer semiconductor structures comprising alternating layers of oppositely doped GaAs and GaAlAs (Journal of Applied Physics, 51(8), August 1980, pp. 4038-4041, and IEEE Transactions on Electron Devices, Vol. ED-29, No. 6, June 1982, pp. 977-984). These structures exhibit bistable electronic switching characteristics similar to those of more conventional pnpn devices. Moreover, these structures may be cleaved and polished in planes which are transverse to the planes of the alternating layers to define several parallel Fabry-Perot cavities. The Fabry-Perot cavities may be operated as parallel coupled lasers by biasing the bistable electronic switching device into its "on-state". The alternating layers serve as alternating confinement and active layers during laser operation, and the direction of optical propagation is parallel to the alternating layers, so the alternating layers do not act as a multilayer interference filter in these laser structures.